1. Field of the Invention
The invention relates to wavelength division multiplexed optical systems generally and, more particularly, to wavelength division multiplexed optical communication systems having tunable optical filters that can rapidly select individual optical channels from a multiplexed optical input signal.
2. Description of the Related Art
Wavelength division multiplexed optical communication systems have rapidly supplanted single channel optical transmission systems for the transportation of voice and data over optical fiber networks. In wavelength division multiplexed (WDM) optical systems, plural optical channels, each channel having a unique optical wavelength, are simultaneously transported over the same optical waveguide (typically a single mode optical fiber). Generally, optical amplifiers are positioned throughout the WDM system for directly increasing the strength of each of the optical channels that comprise the WDM signal without the need to convert the optical channels into electrical signals.
As WDM optical systems proliferate, the complexity of the optical networks increases: optical channels are added, dropped, amplified, switched, terminated, and regenerated. All of these events increase the need for monitoring and control of the individual optical channels that make up the WDM signal. In order to monitor the entire spectrum of the WDM signal, numerous channels must be quickly separated and rapidly measured to develop and accurate “picture” of the WDM signal at a particular moment in time. When large channel counts are involved, on the order of a hundred or more optical channels, it is difficult to quickly scan through the WDM signal and adequately isolate each optical channel to assess the status of the optical network. Such is the case for the so-called “dense” WDM or DWDM in which channel-to-channel spacing is typically less than 1 nanometer.
Various techniques have been developed to examine the spectral position and strength of each of the optical channels that make up the WDM/DWDM optical signal. In one known method a tunable filter is swept” across the spectrum of the WDM optical signal. Since a typical WDM optical signal (in erbium-doped optical amplifier-based systems) can span a range of approximately 80 nanometers, the filter must sweep this entire range to capture each of the optical channels. One type of tunable filter used for this purpose is a Fabry-Perot filter. In this type of filter, two mirrors/reflectors are separated by a cavity. By changing the distance between the mirrors, the cavity size is changed thus altering the wavelength of the optical channel selected by that filter. To change the distance between the mirror elements, an electro-mechanical device, such as a piezoelectric transducer, applies a force to at least one of the mirror elements. Alternatively, the application of thermal energy to change the index of refraction can be used in appropriate material systems to tune the Fabry-Perot filter. Tunable Fabry-Perot filters uses to analyze WDM optical signals are shown in U.S. Pat. No. 6,407,376 to Korn et al. and U.S. Pat. No. 5,408,319 to Halbout et al. Although tunable Fabry-Perot filters such as the ones shown in the patents adequately filter WDM optical signals, the requirement that they be swept across a large spectral range is disadvantageous for WDM optical signals with high channel counts.
Another known technique for creating a tunable filter comprises a fixed demultiplexer (such as an arrayed waveguide grating) followed by a linear array of spatial switches. The demultiplexer takes an input WDM stream and spatially separates the wavelengths. Space switches such as micro-mirrors that are inserted into the paths of the demultiplexed channels can redirect the channel into a specified direction. The switches are mechanically activated. Such filters require several components to be concatenated together and the switch itself is mechanically sensitive. Solid-state switches such as Mach-Zehnder switches can also be used. The number of switches needed grows linearly with the number of channels addressed. Because each channel demultiplexed from a fixed demultiplexer appears at its own distinct spatial port, this port must also be switched to the common output port, increasing complexity and decreasing the performance of the filter.
Another category of tunable filters is the integrated optic delay line filter. In this tunable filter, a series of Mach-Zehnder (MZ) structures is cascaded to give a narrowband filter response. Optionally, the MZs can be integrated onto a dielectric substrate. However this type of filter tends to be very long and becomes longer the narrower the passband becomes. The number of channels over which the filter can be tuned depends linearly on the number of individual MZs. Consequently, this type of filter is not suitable for WDM optical signals having high channel counts. Additionally, each MZ needs to be tuned requiring considerable electrical power. The tuning algorithm to access a channel is complicated, and the filter shape is not the most desirable.
There is a need in the art for improved tunable optical filters that can rapidly select each optical channel in a wavelength division multiplexed optical signal. Such tunable filters could be used for a variety of channel monitoring, demultiplexing, and add-drop devices in WDM optical systems.